For aseptic applications like operations in hospitals the use of sterile tools, instruments or materials is absolutely necessary. At a therefore necessary sterilization the sterilizing agent, for example steam, formaldehyde, ethylene oxide, hydrogen peroxide and/or ozone, is usually transferred via the gas phase to the surface of the instrument to be sterilized, to assure the total kill of existing germs. For this purpose sterilizers with sterilization chambers are normally used, in which the instruments or materials—normally packaged—which have to be sterilized, are put in. For the real sterilization the sterilization chamber is flooded with gaseous sterilant—also named sterilizing agent—, which requires that the air atmosphere inside has to be removed before. The sterilization agent shall contact the surfaces of the instruments or materials to be sterilized so that the desired kill of the germs occurs.
As the complete sterilization of the goods at all surface areas is only guaranteed if the sterilization agent reaches all interior surfaces as well, for example porous goods or hollow devices, the removal of the air inside the goods and inside the sterilization room has to be secured by a suitable air removal procedure at the beginning of the sterilization process. After that the sterilization chamber is flooded with the sterilization agent in order to reach all surfaces of the devices inside the sterilization chamber with the sterilization agent. This is only possible when the complete penetration of the sterilization agent is assured to all surfaces.
The complex structure of medical instruments is known to be problematic in sterilization processes. More and more medical devices are used with relatively long pipes or tubes and comparatively small free cross sections, so that a reliable surface contact of the sterilization agent to all interior surfaces is made more difficult, if there are other gases present. Furthermore materials and goods with complex interior surfaces, for example textile packages, require sterilization. In such cases existing accumulations of remaining air or other non condensable gases (NCG) may prevent complete or part contact of those surfaces. However the complete sterilization is only assured when the air inside the goods is completely removed before the sterilization process, and/or, when during the vacuum stage no air enters through leaks and/or no NCG are introduced into the sterilization chamber with the sterilization agent, to assure, that the sterilization agent can reach all surfaces.
As sterility of instruments cannot be tested directly before use, the validation of sterilization processes after start up and routine monitoring during the sterilization process are necessary. In addition detectors are used, to prove the success of the sterilization process. For example chemical indicators are used which change their colour when their surfaces are covered by the sterilization agent, for example with steam, so that it is recognizable that the chemical indicator has been contacted directly. Alternatively or additionally biological indicators may be used in form of stripes, suspensions or self-contained germ cultures or mixtures of different germ cultures. After the end of a sterilization process it is tested, if all germs have been completely inactivated.
The use of such indicators demonstrates, if an active coverage of the indicator surface with sterilization agent has occurred at the place, where the indicator inside the sterilization chamber is positioned. Using such indicators is no direct proof of the sterilization success at comparatively inaccessible surfaces of complex instruments, because the indicators cannot be placed at those critical areas. Therefore special test devices are sterilized together with the goods which have to be treated, to determine the success of the sterilization. For example, for the sterilization process of textiles or other materials, from Bowie and Dick a standard test pack was described (Bowie, I. W., e.a., The Bowie+Dick autoclave tape test, Lancet I, 1963, p. 585-587), in which a chemical indicator test sheet of DIN A4-size has been placed centrally in a cotton package of 6.6 kg weight. Though this standard test is not exactly reproducible because of the cotton quality, cotton history and individuality of the packages, and its penetration characteristic is different from a hollow device.
Alternatively so called test devices or test device systems can be used. In such a test device system, as described for example in EP 0 628 814 A1 or in EN 867-5, the difficult accessible inside surface of complex instruments is simulated by a suitable model, enabling to monitor the success of the penetration processes into complex instruments in an analogous way.
Those well-known test device systems, also named “process challenge device” (PCD), consist of a suitable detector to approve the contact to the sterilization agent, connected to a suitably chosen length of tube at the gas entering side, which is open at its admission end. This hollow device system simulates the penetration characteristics of similarly designed instruments which are supposed to be sterilized, where especially during an alternate gas exchange according a fractionated vacuum and/or the condensation of steam eventually remaining air or other non-condensable gases at the tube end in the area of the detector are concentrated. So the tube works as a gas-collection-volume for remaining air or other non condensable gases to though the detector is connected to the sterilization chamber via this gas-collection-volume.
If the detector of such a system, connected to the tube end, detects sterilization agent, it can be assumed, that—adding a security supplement according to the Penetration characteristics—the instruments most inaccessible points of their inner surfaces must have been in contact with sterilization agent as well. Such a tube model as a test device which can hold for example biological or chemical indicators as a detector is also intended for the verification of sterilization processes in Euro Standard EN 867-5. To check the sterilization success of more complex goods, test devices of a different construction which are in their dimension suitably adapted, can be used, as described for example in the Euro Standards EN 285, EN 14180, EN 1422 or EN 867-5.
The use of such test devices makes it also possible to use physical test methods under specific conditions. For example from EP 1 172 117 A2 a sterilization test system is known, in which the local change of temperature occurring by the condensation of steam is detected at the place to be monitored as a proof for an occurred contact of an inside surface by the sterilization agent. The test device used in that system is particularly suitable designed regarding to its heat conducting properties.
However the available measure accuracy of these systems is limited. Especially at the modelling of comparably complex or for the sterilization agent hardly accessible instruments the required measure sensitivity is—if at all—only achievable if a comparably voluminous gas-collection-volume is used. Using a tube material to form the gas-collection-volume, heat transfer through the tube wall can appear and may give false results, if thermoelectric measurements are used as detectors.